1. Field of the Invention
The present invention relates generally to measurement-while-drilling tools and specifically to resistivity and dielectric constant measuring tools.
2. Background and Description of the Prior Art
Drilling oil wells is an extremely expensive undertaking. The more confidence that a particular formation will be productive the easier it is for a drilling company to take the financial risks associated with the drilling operation. Technology has progressed to allow acquisition of informative data to determine the viability of a well. Two current methods of formation evaluation are the down hole measurement while drilling (MWD) method and core sample method. In the MWD method, down hole instrumentation measures specified parameters of the formation surrounding the drill stem. In the core sample method, a special drill bit is used that allows the retrieval of a section of formation for evaluation by surface measuring devices. In some cases the instrumentation for MWD is used down hole while also drilling a core sample to measure the properties of the surrounding formation for later comparison to core sample data recorded on the surface.
A technique used for evaluating formations surrounding an earth borehole is to measure the resistivity and dielectric constant of the formation. Porous formations with high resistivity and dielectric constant generally indicate the presence of hydrocarbons while porous formations with low resistivity and dielectric constant are normally water saturated and contain no hydrocarbons.
Typically, tools used to measure these formation characteristics during drilling use a wave propagation tool to measure phase delay and attenuation of electromagnetic waves propagating in the formation over a predefined interval. Such devices are described in detail in U.S. Pat. Nos. 3,551,797 and 4,968,940. All conventional devices for use during drilling use antennas on the outside of a drill collar to transmit or receive the signal. The general direction of advancement is always toward higher data accuracy with higher reliability.
The methods for evaluation of core samples for geophysical parameters which are of interest to the person skilled in the art in the study of oil wells being drilled utilize, for example, the natural radioactivity of the core sample, the absorption of a known radiation emitted by a known source arranged in proximity to the core sample, and the value of the liquid saturation of the core sample (which value is measured by induction).
To date, this type of parameter has been measured and/or detected by first retrieving a core sample and then arranging the core sample which has been withdrawn from the well substantially horizontally on the ground and by moving a carriage equipped with the measuring instrument or instruments manually along the core sample.
Parameters of the abovementioned type can be influenced by the environment of the core sample at the time of measurement, or else similar parameters originating from the environment may be added to the corresponding parameters of the core sample during the measurement taken therefrom. Thus, when the core sample is arranged horizontally, since one of its sides is closer to the ground than the other, this difference in distance may affect the result of the measurement, or else the ground may influence the instruments because of its proximity, this being increasingly so since this proximity is asymmetrical relative to the mass of the core sample as a whole. Overall, lack of accessibility to the core sample makes it difficult to optimize the measurement. The present invention combines the advantages of core sampling with the advantages of MWD to provide an apparatus and method to determine the characteristics of a core sample while the sample is in its original environment and while in its original orientation relative to the surrounding formation.
It is a general objective of the present invention to provide an improved measurement-while-drilling (MWD) tool and method for obtaining highly accurate data regarding resistivity and dielectric constant parameters of solids, liquids or a combination thereof, whether flowing or stationary, while core drilling a well or in a laboratory. This objective can be met with an in-situ core sample measuring embodiment wherein a core sample is measured within the core sample cavity of a drill stem by situating a receiving antenna, a plurality of transmitters and a plurality of receivers in a core drill stem such that the electromagnetic waves propagate through the core sample within the core sample cavity rather than the formation surrounding the drill stem.
This objective may also be met in a laboratory environment with a cylindrical cavity containing the sample to be measured. The cylindrical cavity being equipped with electromagnet wave propagation instrumentation as mentioned in the above drilling system. Other features and advantages will become clear in the following detailed description.